1. Field of the Invention
The present invention relates to solar cells and other semiconductor devices. More particularly, the invention relates to a process for forming ultra-thin semiconductor films for solar cells.
2. Description of the Related Art
Photovoltaic devices, or solar cells, use the specific conductivity properties of semiconductors to convert visible and near visible light energy from the sun into usable electrical energy. In the past, solar cells for use in solar power generation have been formed of single crystal or polycrystalline silicon. However, such solar cells are expensive and difficult to mass-produce, as they require much time and energy for crystal growth and also complex subsequent steps.
Thus, thin film semiconductor solar cells have been developed. Thin film solar cells are typically prepared by forming a thin film layer of a semiconductor compound on a substrate such as a glass or stainless steel sheet, through relatively simple manufacturing steps. Thin film solar cells formed using Group II-VI or Group I-III-VI2 compound semiconductors are particularly desirable, since they can be produced at low cost and with a good mass-production capability, and because films such as CdS and CdTe can be formed uniformly on substantially the entire surface of a glass substrate using a relatively easy process.
However, such thin film solar cells and their manufacturing processes are still in need of further improvement. This is because thin film solar cells are known to have a lower conversion efficiency than silicon crystal photovoltaic devices, and greater manufacturing costs. One preferred process known in the art for the formation of Group II-VI compounds is chemical bath deposition, which has various drawbacks. This process for forming such compounds often exhibits low utilization of the Group II species, leading to the generation of large amounts of hazardous waste by-products, thus increasing processing costs. There is also a problem with particulate formation caused by reaction in the bath, known as homogeneous nucleation, of product species prior to application to the substrate. This leads to imperfections in the resulting film.
Thus, it would be desirable to provide an improved process for forming low cost, high quality thin film solar cells having high conversion efficiencies. It would be further desirable to provide a process which exhibits an increased utilization of Group II species, resulting in the formation of less hazardous waste and thus lowering processing costs. It would also be desirable to limit the formation of particulates formed by homogeneous nucleation of product species, to thereby form uniform, dense films.
The present invention provides a solution to these problems and includes a chemical surface deposition process. The process of the present invention involves the preparation of a liquid coating composition which comprises at least one Group IIB ionic species, at least one Group VIA ionic species, and a complexing agent capable of causing the Group IIB ionic species and the Group VIA ionic species to combine. The liquid coating composition is applied onto a heated substrate surface, which substrate surface is at a temperature higher than the temperature of the liquid coating composition. Prior to application, the solution temperature is maintained at a low enough temperature to reduce or eliminate homogeneous nucleation in the bath. To obtain the desired heterogeneous nucleation on the substrate while minimizing such homogeneous nucleation in the bath, the solution temperature is kept at a lower temperature than the substrate. Application onto the heated substrate causes the Group IIB ionic species and the Group VIA ionic species to react on the substrate surface and form a solid reaction product film of the Group IIB ionic species and the Group VIA ionic species on the substrate. This process solves the problems discussed above by decreasing the formation of particulates formed by homogeneous reactions in bath, dramatically increasing the utilization of Group IIB species (from about 1% to almost 100%), and forming dense, adherent films for thin film solar cells.
The invention provides a chemical surface deposition process for forming a film on a substrate which comprises:
a) forming a liquid coating composition which comprises at least one Group IIB ionic species, at least one Group VIA ionic species, and a complexing agent capable of causing the Group IIB ionic species and the Group VIA ionic species to combine;
b) applying the liquid coating composition onto a heated substrate surface, which substrate surface is at a temperature higher than the temperature of the liquid coating composition; and
c) causing the Group IIB ionic species and the Group VIA ionic species to react on the heated substrate surface under conditions sufficient to form a solid reaction product film comprising a reduced form of the Group IIB ionic species and the Group VIA ionic species on the substrate surface.
The invention also provides a process for forming a solar cell which comprises:
a) forming a liquid coating composition which comprises at least one Group IIB ionic species, at least one Group VIA ionic species, and a complexing agent capable of causing the Group IIB ionic species and the Group VIA ionic species to combine;
b) applying the liquid coating composition onto a heated substrate surface, which substrate surface is at a temperature higher than the temperature of the liquid coating composition;
c) causing the Group IIB ionic species and the Group VIA ionic species to react on the heated substrate surface under conditions sufficient to form a solid reaction product film comprising a reduced form of the Group IIB ionic species and the Group VIA ionic species on the substrate surface;
d) removing any excess liquid coating composition from the substrate;
e) rinsing the film with water; and
f) drying the film.